Seeing earlier arguments for dwarf power, slow pumps, etc., how about this suggestion... separate "Power" into Force and Speed?
WARNING: Potentially tl;dr. In summary, split machine requirements into force minimum, and power-for-full-speed. Use gears to turn low-force high-speed power into high-force low-speed power. Gear systems will need 4 orientations (or 6 to include vertical) for in-out management. Separate from current cornering Gear Assemblies. The sciencey bits are below, including a suggestion for human-gallows power.
All jobs require energy to do. Power is the rate at which energy is expended/generated. Now Energy can be defined by the distance over which a Force is applied, and Speed is the rate at which distance is covered. So, for the equations:
Power = Energy/Time Speed = Distance/Time Energy = Force*Distance
Power = Force*Distance/Time = Force * Speed
What's my point? To have a power level, say 9000 Watts, you could apply the force of a 900 Newton object, say a filthy human in earth gravity, at a constant speed of 10 metres per second. So a human falling at 10 m/s on a rope (preferably by the neck) would provide 9000W of power. You'd get the same power for a 90N object moving at 100m/s, or a large 9000N object at 1m/s, or a huge 90000N object (roughly 9 metric tonnes) at a leisurely 0.1m/s.
You can use gears to switch between these amounts. So while a dwarf walking in a hamster-wheel might go slowly, and not produce very much force (that would depend on weight), it could provide a lot of force even slower using gears (for raising a bridge?) or a small force very quickly (ventilation fans). This could be used to squeeze more versatility out of machinery. Each machine would have a force requirement (to overcome friction) which works much like power now, and if a machine isn't supplied enough force, it won't work, and the system stops. But each machine will still have a power drain, only now, if the total power needs are more than supply, things only slow down, not stop.
Essentially power rating will be the power required for normal speed. Conversely power surplus isn't wasted, it makes things go faster. So a supply of 20 dpu (dwarf power units), supplied at 20dfu (dwarf force, you get the idea) and 1 dsu (speed) would power machines with a total power rating of 40dpu at half speed. If, say, the force requirement was 80 dfu, the machines would stop, but gearing could bring the speed down to 1/4dsu, which would raise force to the right level, and the machines would continue. If more power was then supplied, the gears could allow faster working, until the machines are at full speed, and beyond.
The gears would, like gear assemblies, drain some power. They would also have a minimum force requirement (in). Thus, the balancing act is in the player's hands. Of course, players could construct "gearboxes" out of many differently-configured gears connected to levers. Because you don't want full throttle all the time. ...and sometimes you want "MAXIMUM SPEED!!!!" Just don't stall it.